As a liquid droplet discharging head for discharging liquid droplets, for example, there is known an inkjet head of electrostatically driven system that is mounted in an inkjet recording apparatus. The inkjet head of electrostatically driven system generally includes an electrostatic actuator section composed of an individual electrode (fixed electrode) formed on a glass substrate and a vibration plate (movable electrode) made of silicon arranged opposite to the individual electrode via a predetermined gap. Additionally, it includes a nozzle substrate in which a plurality of nozzle holes for discharging ink droplets are formed, a cavity substrate which is bonded to the nozzle substrate and on which an ink flow path such as an discharging chamber or a reservoir communicating with the above nozzle holes is formed between the nozzle substrate and the cavity substrate. Thereby, the inkjet head is adapted to eject an ink droplet from a selected nozzle hole by applying pressure to the discharging chamber by generating an electrostatic force in the above electrostatic actuator section.
In the conventional electrostatic actuator, for purpose of preventing insulation breakdown and short circuit of an insulation film of the actuator to ensure driving stability and driving durability, the insulation film is formed on opposing surfaces of the vibration plate and the individual electrode. As the insulation film, in general, a silicon thermal oxide film is used. The reason for that is that its manufacturing process is simple and the silicon thermal oxide film has excellent insulation-film characteristics. It is also proposed that, by a plasma CVD (Chemical Vapor Deposition) method, the insulation film made of a silicon oxide film using TEOS (tetraethoxysilane) as a raw gas is formed on the opposing surface of the vibration plate (for example, see Patent Document 1). In addition, in a case of forming the insulation film only on the vibration plate side, residual electric charge is produced inside the insulation film as a dielectric, resulting in reduction in driving stability and driving durability of the actuator. Thus, an electrostatic actuator is proposed in which an insulation film is formed on both of the vibration plate side and the individual electrode side (for example, see Patent Documents 2 and 3). Furthermore, in order to reduce the produced residual electric charge, there is proposed an electrostatic actuator in which electrode protection films made of two layers of films with high- and low-volume resistances are formed only on a surface of the individual electrode side (for example, see Patent Document 4). Moreover, an electrostatic actuator is proposed in which pressure generated by the actuator can be improved by using a dielectric material having a relative permittivity higher than silicon oxide, a so-called High-k material (high permittivity gate insulation film) for the insulation film of the actuator (for example, see Patent Document 5).
[Patent Document 1] Patent Unexamined Patent Application Publication No. 2002-19129.
[Patent Document 2] Patent Unexamined Patent Application Publication No. H8-118626.
[Patent Document 3] Patent Unexamined Patent Application Publication No. 2003-80708.
[Patent Document 4] Patent Unexamined Patent Application Publication No. 2002-46282.
[Patent Document 5] Patent Unexamined Patent Application Publication No. 2006-271183.